GOODWILL - Narrative (Details)	12 Months Ended			360 Months Ended
	Dec. 31, 2017 USD ($) reporting_unit segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) business
Goodwill [Line Items]
Number of segments | segment	2
Number of reporting units | reporting_unit	2
Number of acquisitions of banks or assets of banks | business				25
Goodwill acquired during period	$ 11,000,000
Goodwill impairment	0	$ 0	$ 0
Consumer Banking
Goodwill [Line Items]
Goodwill acquired during period	0
Goodwill accumulated impairment loss	5,900,000,000	5,900,000,000		$ 5,900,000,000
Commercial Banking
Goodwill [Line Items]
Goodwill acquired during period	11,000,000
Goodwill accumulated impairment loss	50,000,000	$ 50,000,000		$ 50,000,000
Western Reserve Partners, LLC
Goodwill [Line Items]
Goodwill acquired during period	$ 11,000,000